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34.2 SAFETY

•Some of the basic rules are,

•a fail-safe design - Programs should be designed so that they check for problems, and shut down in safe ways. Most PLC’s also have imminent power failure sensors, use these whenever danger is present to shut down the system safely.

•proper programming techniques and modular programming will help detect possible problems on paper instead of in operation.

•make the program inaccessible to unauthorized persons

•use predictable, non-configured programs

•use redundancy in hardware

•directly connect emergency stops to the PLC, or the main power supply

•check for system OK at start-up

•provide training for new users and engineers to reduce careless and uninformed mistakes

•use PLC built in functions for error and failure detection

•use well controlled startup procedures that check for problems

•provide clear and current documentation for maintenance and operators

•modular well designed programs

34.2.1 Troubleshooting

•A reasonable troubleshooting guide (note - not debugging),

1.Look at the process and see if it is in a normal state. i.e. no jammed actuators, broken parts, etc. If there are visible problems, fix them and restart the process.

2.Look at the PLC to see which error lights are on. Each PLC vendor will provide documents that indicate which problems correspond to the error lights. Common error lights are given below. If any off the warning lights are on, look for electrical supply problems to the PLC.

HALT - something has stopped the CPU

RUN - the PLC thinks it is OK (and probably is) ERROR - a physical problem has occurred with the PLC

3.Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagree with the physical reality, then interface electronics/mechanics need inspection.

4.Consult the manuals, or use software if available. If no obvious problems exist the problem is not simple, and requires a technically skilled approach.

5.If all else fails call the vendor (or the contractor) for help.